For the reduction of pollutants, in particular for the reduction of nitrogen oxides in exhaust gases of internal combustion engines that operate with excess of air, in particular of diesel engines, various methods have become established for the introduction of reducing fluids (gases or liquids) into the exhaust system of the internal combustion engine.
In particular, for the reduction of nitrogen oxides, SCR (selective catalytic reduction) technology has become established, wherein nitrogen oxides (NOX) contained in the oxygen-rich exhaust gas are selectively reduced with the aid of ammonia (NH3), or a corresponding precursor substance which can be converted into ammonia, to form elementary nitrogen (N) and water (H2O). Here, use is preferably made of aqueous urea solutions. The urea solution is hydrolyzed by means of a hydrolysis catalytic converter, or directly on the SCR catalytic converter, to form ammonia and carbon dioxide. For this purpose, the urea solution is injected into the exhaust-gas stream upstream of the hydrolysis catalytic converter or of the SCR catalytic converter by means of special dosing systems. Here, reliable supply and precise dosing of the reducing agent must be ensured.
In the event of underdosing of the reducing agent, efficient removal of nitrogen oxides from the exhaust gas is not possible. On the other hand, overdosing of reducing agent can result in undesired emissions, for example of ammonia—a so-called breakthrough of reducing agent.
To attain the highest possible rate of conversion of the nitrogen oxides, there is generally a need for the reducing agent that is used to be dosed in a precise manner in accordance with demand, wherein a breakthrough of reducing agent should as far as possible be prevented.
In the case of liquid reducing agent such as the customary aqueous urea solutions, the dosing can be performed by means of a dosing valve or injector. The actuation time and thus the opening time of the injector are in this case definitive of the amount of reducing agent supplied to the exhaust-gas aftertreatment system.
In the case of modern, air-free SCR systems, ever higher injection pressures are used. For the generation of the corresponding pressure, the injector is connected via a line to a reducing agent pump. A higher injection pressure has the advantage that the atomization is finer and thus the ammonia can be more easily released from the normally aqueous reducing agent solution.
On the other hand, owing to the ever higher pressures, the risk of leakage at the injector, or leaks occurring in the line system between the outlet of the reducing agent pump and the injector, is increased. Since the reducing agent pump is generally arranged close to or in a tank which stores the reducing agent, and the injector is assigned to the exhaust-gas aftertreatment system, there are resulting long line distances, in some cases of several meters, between the reducing agent pump and injector, such that, owing to aging and mechanical loading as a result of arrangement normally along the underbody of the vehicle, the risk of leakage in this line branch, which is normally formed from plastic hoses, is increased. Also, the injector is subject to increased loads owing to its exposed position in the exhaust tract of the internal combustion engine. A situation may for example arise in which the nozzle of the injector no longer fully closes owing to coking, and thus correct metering of the reducing agent is no longer ensured.
Since such leaks have a direct effect on the pollutant emissions of the vehicle, said components must be checked with regard to correct functioning.
DE 10 2009 014 809 B3 discloses a method and a device for checking an injection device for introducing a catalytically oxidizable reducing agent into an exhaust tract of an internal combustion engine at a location upstream of an oxidation catalytic converter and of a particle filter. In the overrun cut-off operating state of the internal combustion engine, in which the fuel injection of the internal combustion engine is deactivated and no regeneration of the particle filter is demanded, the exhaust-gas temperature downstream of the oxidation catalytic converter is detected and stored. The injection device, which is held closed, is subjected, for a predefined time period, to a pressure which is higher than the pressure during the injection of the reducing agent with the injection device open. After the time period has elapsed, the exhaust-gas temperature downstream of the oxidation catalytic converter is detected, and the two exhaust-gas temperatures are compared with one another. The injection device is then evaluated with regard to its leak-tightness in a manner dependent on the result of the comparison. The injection device is regarded as being defective if the value of the exhaust-gas temperature after the pressure increase is higher than the value of the exhaust-gas temperature before the pressure increase.
DE 10 2010 029 852 A1 describes a method for the diagnosis of an exhaust-gas purification device, which can be controlled by means of a control signal, for the purification of an exhaust-gas stream of an internal combustion engine by means of selective catalytic reduction. To permit improved diagnosis, it is provided that:                a volume flow of a reducing agent required for the selective catalytic reduction is delivered by means of a pump device to a dosing device which is connected downstream of the pump device and assigned to the exhaust-gas stream,        the control signal of the exhaust-gas purification device is changed or is adjusted in a defined manner,        a reaction of a pressure of the volume flow between the pump device and the dosing device to the change in the control signal is determined, and        the reaction is interpreted for the purposes of diagnosing the exhaust-gas purification device.        